In Learning to Bow (page 72) Bruce Feiler repeats the oft-used argument that is comes from the nature-connected animism of Shinto, but I’ve seen much more convincing arguments that it comes directly from the Chinese court, and ditto with appreciating cherry blossom. I do like his other ideas of the Japanese making a “way” out of it and just generally being as “gambaru” about it as they are about work etc though.